In the course of manufacture of semiconductor devices, especially integrated circuits, a series of photomasking steps are conducted for defining prescribed areas or regions on a semiconductor wafer whereat respective patternings of the wafer and the formation of successive layers of circuit defining material are conducted. At each pattern-defining step, a respective photomask must be precisely aligned with the wafer in order to accurately fix the locations of the patterns through which the integrated circuit is formed. Usually, this alignment procedure involves the use of visual keys on the wafer and the mask which are optically aligned with one another through the use of a microscope/TV monitor apparatus. For an overview of a variety of alignment schemes, particularly photomask/wafer alignment mechanisms, attention may be directed to the U.S. Pat. Nos. to Locombat 4,402,610; Wilczynski 4,232,969; Kleinknecht et al 4,211,489; Koizumi et al 4,153,371; Feldman et al 4,037,969; and Pekelsky 4,459,026.
In general such systems are configured in the manner illustrated in FIG. 1, which shows a block diagram of a conventional photomask alignment apparatus. In order to optically view and align the wafer and photomask alignment keys, a beam of light T from a monochromatic light source 10 is projected via a projection/return optical unit 11 through a window 12W in a photomask 12 and, via reduction optics 13, onto that portion of the surface of semiconductor wafer structure 14 being processed containing the alignment mark 14P. The light is then reflected from the alignment mark 14P on the wafer 14 and returns as a return beam R through the photomask window 12W and projection/return unit 11, to be viewed by an alignment image monitor apparatus 15, usually containing a TV monitor coupled to the output of a microscope, so that the system operator is presented with an image of the superposition of the alignment mark 14P on the wafer 14 and the alignment window 12W formed in the photomask reticle 12. From this composite image of the photomask window 12W and alignment mark 14P, the operator seeks to control the relative positions of the photomask 12 and wafer 14 (e.g. through an X-Y table 18 driven by an X-Y alignment control unit 17 in response to an alignment control input coupled over link 16).
Unfortunately, the composite image of the alignment mark 14P on the wafer 14 and the window 12W in the photomask 12, as monitored by imaging apparatus 15 and viewed by the operator, suffers from a variation in clarity and contrast during the sequence of photomask/processing steps to which the wafer structure is subjected for manufacturing the intended integrated circuit. This degradation in image quality has been found to be a result of the use of a monochromatic imaging beam for viewing the alignment key or mark on the semiconductor wafer. A typical light source through which alignment of the photomask and the wafer is carried out may have a frequency on the order of 440 nanometers (.+-.10-30 nm), so that the light source 10 in the configuration shown in FIG. 1 is effectively monochromatic. As successive layers are formed on wafer 14 through the photomask processing sequence, the monochromatic light beam is subjected to multiple reflections and absorption through the laminate of layers on the wafer surface. Because of this effect by the wafer structure on the imaging beam, the image of the alignment mark 14P on the wafer 14 contained within the composite image of the window 12W and the alignment mark 14P as viewed by the monitor apparatus 15 may suffer a substantial reduction in signal-to-noise ratio, whereby precise location of the alignment mark within the photomask window is extremely difficult or practically impossible to obtain, for the degree of alignment tolerance required. As a result, processing of the wafer through that photomask may not necessarily achieve patterning to the precision necessary for achieving the intended configuration and tolerances of the sought-after integrated circuit.